Experimental and numerical investigation on the laminar flame speed of CH₄/O₂ mixtures diluted with CO₂ and H ₂O

The effects of CO₂ and H₂O addition on premixed oxyfuel combustion are investigated with experiments and numerical simulations on the laminar flame speed of CH₄/O₂/CO₂/H ₂O(v) and CH₄/O₂/N₂/H ₂O(v) mixtures, at atmospheric pressure and for a reactants inlet temperature Tu = 373 K. Experiments are conducted with steady laminar conical premixed flames over a range of operating conditions representative of oxy-fuel combustion with flue gas recirculation. The relative O₂-to-CO ₂ and O₂- to-N₂ ratios, respectively defined as O₂/(O₂+CO₂) (mol.) and O₂/(O ₂+N₂) (mol.), are varied from 0.21 to 1.0. The equivalence ratio of the mixtures ranges from 0.5 to 1.5, and the steam molar fraction in the reactive mixture is varied from 0 to 0.45. Laminar flame speeds are measured with the flame area method using a Schlieren apparatus. Experiments are completed by simulations with the PREMIX code using the detailed kinetic mechanism GRI-mech. 3.0. Numerical predictions are found in good agreement with experimental data for all cases explored. It is also shown that the laminar flame speed of CH₄/O₂/N₂ mixtures diluted with steam H₂O(v) features a quasi-linear decrease when increasing the diluent molar fraction, even at high dilution rates. Effects of N₂ replacement by CO₂ in wet reactive mixtures are then investigated. A similar quasi-linear decrease of the flame speed is observed for CH ₄/O₂/CO₂ H₂O-diluted flames. For a similar flame speed in dry conditions, results show a larger reduction of the burning velocity for CH₄/O₂/N₂/H₂O mixtures than for CH₄/O₂/CO₂/H₂O mixtures, when the steam molar frac- tion is increased. Finally, it is observed that the laminar flame speed of weakly (CO₂, H₂O)-diluted CH₄/O₂ mixtures is underestimated by the GRI-mech 3.0 predictions.